It is known that a liquid such as water can be rapidly cooled by injecting a cryogenic liquid, generally liquid nitrogen into it. The liquid nitrogen extracts heat from the water and vaporises. The cold nitrogen vapour extracts further heat from the water during the period of time in which the two fluids are in contact with one another. The water is thereby further cooled. There are two well known problems that need to be overcome before this method of cooling can be performed to a reasonable level of efficiency. The first problem is to prevent local freezing of the water (or other liquid) from obstructing a nozzle or other device used to introduce the liquid nitrogen into the water. The second problem is to find a means whereby a reasonable proportion of the refrigerative capacity of the liquid nitrogen is extracted therefrom by the time the resulting vaporised nitrogen disengages itself from the water being cooled. This problem is not an easy one to solve since upon injection into the water the liquid nitrogen will tend to vaporise instantaneously with a concomitant large increase in volume. (One volume of liquid nitrogen yields on vaporisation more that 600 volumes of gas.) The so-formed nitrogen vapour thus tends to pass rapidly out of contact with the water with the result that very little time is available to effect heat exchange between the nitrogen vapour and the water. These problems become particularly significant if it is desired to operate the process continuously.
It has been proposed to overcome the first problem, that is blocking of a liquid nitrogen injection nozzle with ice, by either heating the nozzle or passing warm gas through it. This solution, however, provides no help in improving the heat transfer between the water and the liquid nitrogen and makes continuous operation difficult.
It is accordingly an aim of the present to provide a process and apparatus for cooling a liquid by the direct introduction into the liquid of the cryogen (by which is meant a cryogenic liquid) which ameliorates the difficulties discussed above.